dmaengine: stm32-mdma: Use bitfield helpers

Use the FIELD_{GET,PREP}() helpers, instead of defining custom macros
implementing the same operations.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/36ceab242a594233dc7dc6f1dddb4ac32d1e846f.1637593297.git.geert+renesas@glider.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>